A bus carrying Belgian students crashed into a tunnel wall in Switzerland, killing 28 people, 22 of which were children.

­Another 24 students were taken to hospital with injuries, local police say.

The bus was carrying 12-year-olds from two schools back from a ski holiday in the southwestern region of Valais when it crashed on Tuesday evening. The cause of the accident has yet to be determined.

The bus was not speeding at the time of the accident, nor was other vehicle involved, local prosecutor Olivier Elsig told reporters Wednesday. He also said that all the children on the bus were wearing seat belts.

Police reported that the bus veered, hit a curb and then crashed into a concrete wall. The front of the vehicle was heavily damaged by the impact, leaving people trapped inside.

The highway was closed for traffic as the rescue operation was underway, AP reported. Some eight helicopters and a dozen ambulances took victims to the hospital. Dozens of firefighters and police, 15 doctors and three psychologists were also called to the scene.

Belgium has announced a day of national mourning after the tragic news broke. The country’s Prime Minister Elio Di Rupo is to travel to Switzerland, local RTBF TV channel reports. The government has offered all necessary assistance to the victims and their families.

According to Belgian media, the children who suffered in the crash came from two towns in Flanders.

A helicopter takes off from the entrance of a tunnel into which a foreign coach crashed in Sierre (REUTERS/Denis Balibouse)

Rescuers carry a stretcher to an emergency helicopter at the entrance of a tunnel into which a foreign coach crashed in Sierre (REUTERS/Denis Balibouse)

Emergency services work on the wreckage of a bus that crashed into a motorway tunnel in Sierre (REUTERS/Handout)

The wreckage of a bus that crashed into a motorway tunnel is pulled in Sierre, western Switzerland (REUTERS/Denis Balibouse)

The wreckage of a bus that crashed into a motorway tunnel is pushed into a warehouse in Sierre, western Switzerland (REUTERS/Denis Balibouse)